The present invention relates to gear assemblies in general and, more specifically, to a differential gear assembly which includes a gear case, a pair of drive axles received in bores formed in the sides of the case, and a differential gear arrangement mounted centrally in the case for driving the axles. The case for the differential assembly may include a flange for receiving a ring gear or other means for providing power input to the differential from the drive shaft of the vehicle in a conventional manner.
Conventional differential assemblies utilize four interengaged bevel gears that rotate about two orthogonal axes. In contrast, the differential assembly which is associated with this invention, is of the type that does not utilize bevel gears, and is generally of the design shown in U.S. Pat. No. 2,859,641 issued Nov. 11, 1958 in the name of Gleasman. This patent is incorporated herein by reference to the extent necessary to provide specific details of the structure of the assembly. This type of differential (hereinafter referred to as a "worm-type differential") includes worm or side gears, coupled to each axle end as drive gears, together with so called balancing or transfer gears associated with each of the side gears and in mesh with each other for transferring and dividing torque between the axle ends. The transfer gears are mounted in pairs, and each transfer gear of a pair rotates on an axis of rotation that is substantially parallel to a tangent of the envelope of an associated axle drive gear.
The transfer gears are in reality combination gears, i.e., the middle portion of each gear constitutes a worm wheel portion while the outer ends of the gear are formed with integral spur gear portions. The arrangement is such that, for any given pair of combination gears, the worm wheel portion of a first combination gear meshes with one side gear while the worm wheel portion of a second combination gear meshes with the other side gear, and the spur gear portions of the respective combination gears mesh with each other.
In one example of the worm-type differential assembly, a set of three combination gears are arranged substantially in a first single plane at approximately 120.degree. intervals about the periphery of each side gear, each of the three combination gears being paired with a combination gear of a second set of three combination gears similarly arranged with respect to the second side gear in a second single plane parallel to the first plane.
Because of the substantially planar arrangement of each set of combination gears about the circumference of each side gear, a single zone of contact is established about the surface of each side gear at the meshing point between the combination gears of each set and its respective side gear. In this zone, Hertzian stresses are concentrated leading to possible surface deterioration and wear along the relatively short and narrow contact path and, ultimately, to shortened differential life.
It is a principal object of this invention to improve the durability life of the side gears and combination gears in a worm-type differential assembly. In the present invention, the mounting locations of at least one pair of combination gears is staggered or offset with respect to the mounting locations of the remaining pair of combination gears in a direction substantially parallel to the axes of rotation of the side gears. As a result, unit pressure loads are dispersed over a larger zone of contact on each side gear, heat dissipation is improved, surface deterioration is distributed over a greater area of the side gears and lubricant life is extended. Consequently, the durability and functional life of the entire differential assembly is also improved.
While the exemplary embodiment is particularly designed for application with the worm-type differential, similar benefits may accrue in any gear assembly where a plurality of worm wheels or gears presently mesh with a single worm or helical gear along a single contact path.